1. Field of the Invention
The present invention relates to a differential refractometer which is employed as a detector for an analyzer, and a liquid chromatograph which is provided with such a differential refractometer as a detector.
2. Description of the Background Art
A differential refractometer is provided with a flow cell having two cells divided by a partition wall which is inclined with respect to an optical axis of a measuring beam. A sample solution passes through one of the cells, while a reference solution passes through or is held in the other cell. The differential refractometer further comprises a photodetector for receiving the measuring beam which is transmitted through and refracted by the flow cell, and an optical system for applying the measuring beam to the flow cell through a slit and guiding the measuring beam passing through the flow cell to the photodetector so that a slit image is formed on the photodetector. Change in refractive index of the sample solution is detected from the amount of displacement of the slit image on the photodetector.
As shown in FIG. 5A, a conventional differential refractometer is provided with a photodetector which is divided into two portions 2-1 and 2-2 by a straight line 4 being perpendicular to a direction X of movement of a slit image 6, to detect displacement of the slit image 6 on the photodetector. As the slit image 6 is moved toward the photodetector portion 2-2 upon increase in refractive index of a sample solution, a signal processing circuit performs the following operation and outputs the result: EQU S=C.multidot.(s.sub.2 -s.sub.1)/(s.sub.2 +s.sub.1)
where s.sub.1 and s.sub.2 represent detection outputs of the photodetector portions 2-1 and 2-2 respectively, and C represents a constant. Assuming that the refractive index of the sample is changed by .DELTA.n and the slit image 6 is moved in the direction X by a distance .DELTA.x, the above equation is transformed as follows: ##EQU1## where c represents a span constant, L represents the distance between a flow cell and the photodetector, and d represents the width of the slit image 6. It is assumed that a partition wall of the flow cell is inclined at an angle of 45.degree.. Thus, the output is proportional to the change in refractive index of the sample. On this photodetector, the signal is disadvantageously saturated if the overall slit image 6 enters the photodetector portion 2-2 as shown in FIG. 5B. In starting of measurement, therefore, it is necessary to locate the slit image 6 on a central portion across the straight line 4 separating the photodetector.
The differential refractometer is employed as a detector for an analyzer such as a liquid chromatograph, which is applied to two purposes of analysis and preparative use. When a highly sensitive differential refractometer for analysis is applied to preparative use, its signal is disadvantageously saturated due to a sample of relatively high concentration flowing a flow cell. In order to cope with this problem, therefore, generally employed is a flow cell having a partition wall which is varied in angle of inclination with analysis and preparative use. As an angle formed by the partition wall of such a flow cell and an optical axis of a measuring beam approaches 90.degree., the distance .DELTA.x of movement of a slit image is reduced with respect to the same change in refractive index, to lower sensitivity.
A UV (ultraviolet) detector is also employed as a detector for a liquid chromatograph. Such a UV detector is provided with cassette type flow cell so that a flow cell for analysis can be easily replaced by that for preparative use or vice versa. In the differential refractometer, however, the overall optical system including the flow cell is temperature-controlled and hence it is difficult to replace the flow cell. The differential refractometer is also applicable to both purposes of analysis and preparative use since its sensitivity can be varied with the angle of the partition wall dividing the cells. In general, however, different chromatographs are independently applied to analysis and preparative use since it is difficult to replace the flow cell. In order to solve this problem, Japanese Patent Laying-Open Gazette No. 63-27733 (1988) describes a detector comprising a light emitting diode serving as a light source, a condenser lens which is interposed between this light source and a slit, a flow cell which is divided into a sample cell and a reference cell having independent passages respectively, and a PSD (semiconductor position detector) for serving as a photodetector portion. However, the PSD employed in this detector is unsuitable for analysis due to a large noise.
A flow cell of a differential refractometer comprises a sample cell and a reference cell. A narrow pipe of 0.25 to 0.5 mm in inner diameter is provided as a conduit on an inlet side of the sample cell, to prevent reduction of separation by its column. On the other hand, an outlet side of the sample cell and inlet and outlet sides of the reference cell are not particularly restricted and conduits therefor are prepared from pipes which are larger in thickness than that provided on the inlet side of the sample cell.
In order to apply a differential refractometer to two purposes of analysis and preparative use, the following two points are required:
(a) A detection signal must not be saturated up to a high concentration region since a sample of high concentration flows into the cell at a high velocity in preparative use. PA1 (b) Thick pipes must be provided on inlet and outlet sides of the cells in preparative use.
While it is possible to satisfy the point (b) by changing connection of flow lines so that a cell serving as a reference cell in analysis in turn serves as a sample cell in preparative use, it is impossible to satisfy the point (a). As the result, different detectors are independently required for analysis and preparative use. Thus, a detector having a partition wall which is inclined at an angle of 45.degree. is applied to analysis, while that having a partition wall which is inclined at an angle of 6.degree. to 12.degree. is applied to preparative use.